The present invention relates generally to power converters for lighting control systems. More particularly, the invention as disclosed herein relates to dimming current control for high power factor, constant current buck-boost converters.
Buck-boost converters are conventionally very good candidates for use with wide range input voltage (120-277V), high power factor non-isolated constant current LED drivers. Such converters are relatively low cost and compact in nature. A typical topology, as represented for example in FIG. 1, has a drawback in that the output does not share the same ground as the control IC. This makes the current control very complicated.
For a conventional LED driver circuit 10 as shown in FIG. 1, V1 is the input AC source. L1 is a common mode inductor for electromagnetic interference (EMI). Capacitor C1 is an EMI filter capacitor. Inductor L2 is a differential EMI inductor. Diodes D1-D4 are input rectifier diodes for converting the AC input supply to a DC power supply. Capacitor C2 is a high frequency filter capacitor for the converter. Resistors R1 and R2 define a voltage divider coupled across filtering capacitor C2. Inductor L3 is a buck-boost inductor that stores that energy and releases it according to the control of IC. MOSFET Switch Q1 is a switching element that is controlled by driver signals generated from the IC. Diode D5 is a rectifier diode that bypasses the current from the primary winding L3p of the buck-boost inductor to output capacitor C4 when the switching element Q1 is off.
The controller IC as shown in FIG. 1 typically can be a power factor control (PFC) controller IC as is known in the art, such as for example the L6562 offered by STMicroelectronics. The controller IC has a MULT pin that senses the input line signal via a node between the voltage dividing resistors R1 and R2. The controller IC also has a zero current detection (ZCD) pin that is coupled to a secondary winding L3s of the buck-boost inductor via resistor R3, wherein the controller IC may ensure transition mode operation by controlling the turn on time of the switching element Q1. The controller IC also has an Isense pin that senses the current going through the switching element Q1 and resistor R5. The controller IC further includes an internal op amp with a Vsense input and COMP as output. C3 is an integration capacitor for the control loop.
Typically, there is an internal voltage reference in the controller IC which is used as a control reference. The controller IC compares this internal reference with the external Vsense signal to tightly control the output. For constant current control, Vsense needs to be a current feedback signal that comes from the load.
However, the controller IC does not share the same ground as the output load, as shown in FIG. 1. As a result, an expensive isolated signal coupler is typically required to transfer the real current sensing signal from the output stage to the IC stage. Resistor R6 is the load current sensing resistor.
This isolated signal coupler is not only expensive, but also introduces error and complicates the control scheme. Therefore, it would be desirable to eliminate this type of isolated signal coupler in a buck-boost converter topology.
It would further be desirable to have a dimming control circuit that has the same ground as GNDmain so that only one dimming signal is required in order to control multiple channels of a buck-boost converter.